Stomatal pores in the epidermis of plant leaves enable the control of plant water loss and the influx of CO2 into plants from the atmosphere. Carbon dioxide is taken up for photosynthetic carbon fixation and water is lost through the process of transpiration through the stomatal pores. Each stomate is made up of a specialized pair of cells named guard cells, which can modify the size of the stomatal pore by controlling guard cell turgor status. An important trait in agriculture, in biotechnological applications and the production of biofuels is the water use efficiency of plants. The water use efficiency defines how well a plant can balance the loss of water through stomata with the net CO2 uptake into leaves for photosynthesis and hence its biomass accumulation. Several biotic and abiotic factors influence the state of stomatal opening thereby optimizing the water use efficiency of a plant in a given condition. The concentration of CO2 regulates stomatal movements, where high levels of CO2 will lead to stomatal closing and low levels of CO2 will induce stomatal opening. Thus CO2 regulates CO2 influx into plants and plant water loss on a global scale. However, at present no CO2 sensors have been identified. Knowledge on the CO2 receptors that regulate CO2 responses could be used to manipulate the CO2 response so that the water use efficiency during plant growth could be enhanced through engineering.
How plants sense the level of carbon dioxide (CO2) has remained unknown. Knowledge of how CO2 is perceived by a plant could be used to manipulate the CO2 response so that the carbon and water use efficiency during plant growth could be enhanced.
Phosphoenolpyruvate (PEP) Carboxylase (PEPC; EC 4.1.1.31) is a key enzyme of photosynthesis in those plant species exhibiting the C4 or CAM pathway for CO2 fixation. The principal substrate of PEPC is the free form of PEP. PEPC catalyzes the conversion of PEP and bicarbonate to oxalacetic acid inorganic phosphate (Pi). This reaction is the first step of a metabolic route known as the C4 dicarboxylic acid pathway, which minimizes losses of energy produced by photorespiration. PEPC is present in plants, algae, cyanobacteria, and bacteria.
Carbon fixation, or the conversion of CO2 to reduced forms amenable to cellular biochemistry, occurs by several metabolic pathways in diverse organisms. The most familiar of these is the Calvin Cycle (or “Calvin-Benson” cycle), which is present in cyanobacteria and their plastid derivatives, such as chloroplasts, and proteobacteria. The Calvin cycle utilizes the enzyme “rubisco”, or “ribulose-1,5-bisphosphate carboxylase/oxygenase”. Rubisco exists in at least two forms: form I rubisco is found in proteobacteria, cyanobacteria, and plastids, e.g., as an octo-dimer composed of eight large subunits, and eight small subunits; form II rubisco is a dimeric form of the enzyme, e.g., as found in proteobacteria. Rubisco contains two competing enzymatic activities: an oxygenase and a carboxylase activity. The oxygenation reaction catalyzed by Rubisco is a “wasteful” process since it competes with and significantly reduces the net amount of carbon fixed. The Rubisco enzyme species encoded in various photosynthetic organisms have been selected by natural evolution to provide higher plants with a Rubisco enzyme that is substantially more efficient at carboxylation in the presence of atmospheric oxygen.